16 Scheduling

McGraw-Hill/Irwin Copyright © 2007 by The McGraw-Hill Companies, Inc. All rights reserved.

Learning Objectives

   Explain what scheduling involves and the importance of good scheduling. Discuss scheduling needs in high-volume and intermediate-volume systems. Discuss scheduling needs in job shops.

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Learning Objectives

 Use and interpret Gantt charts, and use the assignment method for loading.  Discuss and give examples of commonly used priority rules.  Describe some of the unique problems encountered in service systems, and describe some of the approaches used for scheduling service systems.

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Scheduling

 Scheduling: Establishing the timing of the use of equipment, facilities and human activities in an organization  Effective scheduling can yield  Cost savings  Increases in productivity

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High-Volume Systems



Flow system

: High-volume system with Standardized equipment and activities 

Flow-shop scheduling

: Scheduling for high volume flow system

Work Center #1 Work Center

#2

Output 16-5

Scheduling Manufacturing Operations

High-volume Intermediate volume Low-volume Service operations

Build A A Done Build B B Done Build C C Done Build D Ship

JAN FEB MAR APR MAY JUN On time!

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High-Volume Success Factors

 Process and product design  Preventive maintenance  Rapid repair when breakdown occurs  Optimal product mixes  Minimization of quality problems  Reliability and timing of supplies

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Intermediate-Volume Systems

 Outputs are between standardized high volume systems and made-to-order job shops  Run size, timing, and sequence of jobs  Economic run size:

Q

0  2

DS H p p



u

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Scheduling Low-Volume Systems



Loading

- assignment of jobs to process centers 

Sequencing

- determining the order in which jobs will be processed 

Job-shop scheduling

 Scheduling for low-volume systems with many variations in requirements

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Gantt Load Chart

Figure 16.2



Gantt chart

- used as a visual aid for loading and scheduling Work Center 1 2 3 4 Mon.

Job 3 Job 1 Job 10 Tues. Wed. Thurs.

Job 3 Job 7 Job 4 Job 6 Fri.

Job 7

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Loading



Infinite loading

– jobs are assigned to work centers without regard for the capacity of the work center.



Finite loading

– jobs are assigned to work centers taking into account the work center capacity and job processing times  Vertical loading  Horizontal loading

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Loading (cont’d)



Forward scheduling

– scheduling ahead from some point in time.



Backward scheduling

– scheduling by working backwards in time from the due date(s).



Schedule chart

– a form of Gantt chart that shows the orders or jobs in progress and whether they are on schedule.

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Sequencing



Sequencing

: Determine the order in which jobs at a work center will be processed.



Workstation

: An area where one person works, usually with special equipment, on a specialized job.

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Sequencing



Priority rules

: Simple heuristics used to select the order in which jobs will be processed.

Everything #1 Priority is



Job time

: Time needed for setup and processing of a job.

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Priority Rules

Table 16.2

 FCFS - first come, first served  SPT - shortest processing time  EDD - earliest due date  CR - critical ratio  S/O - slack per operation  Rush - emergency

Top Priority 16-15

Assumptions of Priority Rules

    The setup of jobs is known Setup time is independent pf processing sequence Setup time is deterministic There will be no interruptions in processing such as:   Machine breakdowns Accidents  Worker illness

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Example 2

Table 16.4

Rule FCFS SPT EDD CR Average Flow Time (days) 20.00

18.00

18.33

22.17

Average Tardiness (days) 9.00

6.67

6.33

9.67

Average Number of Jobs at the Work Center 2.93

2.63

2.68

3.24

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Two Work Center Sequencing



Johnson’s Rule

: technique for minimizing completion time for a group of jobs to be processed on two machines or at two work centers.

 Minimizes total idle time  Several conditions must be satisfied

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Johnson’s Rule Conditions

 Job time must be known and constant  Job times must be independent of sequence  Jobs must follow same two-step sequence  Job priorities cannot be used  All units must be completed at the first work center before moving to second

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Johnson’s Rule Optimum Sequence

1.List the jobs and their times at each work center 2.Select the job with the shortest time 3.Eliminate the job from further consideration 4.Repeat steps 2 and 3 until all jobs have been scheduled

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Scheduling Difficulties

    Variability in     Setup times Processing times Interruptions Changes in the set of jobs No method for identifying optimal schedule Scheduling is not an exact science Ongoing task for a manager

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Minimizing Scheduling Difficulties

 Set realistic due dates  Focus on bottleneck operations  Consider lot splitting of large jobs

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Theory of Constraints

   The Theory of Constraints Goal is to maximize flow through the entire system Emphasizes balancing flow Improve performance of bottleneck:    Determine what is constraining the operation   Exploit the constraint Subordinate everything to the constraint Determine how to overcome the constraint Repeat the process for the next constraint

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Theory if Constraints Metrics

   Throughput – the rateat which the system generates money through sales Physical assets – the total system investment   Inventory Buildings and land  Plant and equipment Operating expense – money the system spends to convert inventory into throughput

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Scheduling Services Considerations

 Scheduling services different from manufacturing   Inability to store or inventory services Random nature of customer requests for service  Point of customer contact  Back-office operations  Front-office operations

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Scheduling Services

 Appointment systems  Controls customer arrivals for service  Reservation systems  Estimates demand for service  Scheduling the workforce  Manages capacity for service  Scheduling multiple resources  Coordinates use of more than one resource

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Yield Management



Yield Management

– the application of pricing strategies to allocate capacity among various categories of demand.

 The goal is to maximize the revenue generated by the fixed capacity  Fixed capacity  Hotel, motel rooms  Airline seats  Unsold rooms or seats cannot be carried over

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Cyclical Scheduling

 Hospitals, police/fire departments, restaurants, supermarkets  Rotating schedules  Set a scheduling horizon  Identify the work pattern  Develop a basic employee schedule  Assign employees to the schedule

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Service Operation Problems

   Cannot store or inventory services Customer service requests are random Scheduling service involves  Customers  Workforce  Equipment

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